The Propensity of European Grayling to Migrate Is Determined by Individual Characteristics

Abstract

Migrations are characterised as adaptive mechanisms aimed at improving the fitness of an individual. Hence, the propensity to migrate can be analysed by considering body size and movement activity when individuals search for available resources. We studied the relationships among individual traits, movement activity and migration of the European grayling Thymallus thymallus (Linnaeus 1758) in a native riverine environment during a one-year-long field study. We observed the diel movement activity of 37 individuals along the longitudinal profile of the river using radio tags with temperature sensors. The propensity to migrate differed among the tagged individuals. The fish that maintained positions over the entire annual period and whose dispersal did not exceed 2 km from their original positions were considered a stationary group. The fish migrating downstream from their summer positions to wintering positions and returning to their original positions the following year were considered a migratory group. Long-distance downstream migrations to wintering positions mostly started in early autumn, and the fish reached the farthest positions, up to 30 km apart, in December and January. Upstream migration started at the end of February and was most intensive in March and April. The highest upstream positions were reached by the graylings in summer. These findings demonstrate that fish are motivated to migrate not only to reach spawning areas in spring but also to search for food and shelter resources in summer. Over 24 h periods, the positions along the upstream–downstream axis, movement and body temperature of the migratory and stationary fish differed, indicating their distinct habitat preferences during the diel cycle. Body mass appeared to predict the migration of three-year-old females and males, with a greater migratory propensity for females than males. Our results suggest different behavioural strategies within the population of European grayling, that is, differences between stationary fish and fish migrating long distances among wintering, spawning and feeding areas.